1. Field of the Invention
This invention relates to a method of bonding workpieces and, more particularly, to a method of simultaneously bonding at least two dissimilar workpieces to a third workpiece, and to an article adapted for use with the method.
2. Description of the Prior Art
It is well known that two metallic workpieces can be bonded together by positioning the workpieces against each other and applying bonding energy to the abutting workpieces in the form of mechanical pressure and either thermal or ultrasonic energy. It is also well known that workpieces of some materials can be bonded together solely by applying mechanical pressure, where the pressure is sufficient to significantly deform at least one of the workpieces. More typically, however, a combination of mechanical pressure and either thermal or ultrasonic energy is used.
In compliant bonding (as disclosed in U.S. Pat. Nos. 3,533,155, 3,650,454, 3,669,333, 3,625,783, and 3,655,177), a compliant medium, such as aluminum, is placed between a bonding tool and a workpiece to be bonded. Typically, several smaller workpieces, such as electronic device leads, are to be simultaneously bonded to a larger workpiece, such as a circuit substrate. When mechanical pressure and, if necessary, thermal or ultrasonic energy are applied to the compliant medium, the compliant medium deforms around the smaller workpieces, thus limiting the clamping pressure applied to each smaller workpiece to that pressure necessary to deform the compliant medium around the smaller workpieces. Compliant bonding is particularly useful for simultaneously bonding multiple smaller workpieces to a larger workpiece because the compliant medium regulates the pressure applied to each smaller workpiece, thereby compensating for dimensional or positional irregularities in the smaller workpieces and the larger workpiece.
Electronic devices are often assembled by multiple bonding steps. For example, relatively thin leads on a beam-lead semiconductor device may be bonded to a substrate in a first step and relatively thick leads for connection to external circuits may be bonded to the substrate in a second step. Because of the different thicknesses and material properties of the device leads and the external leads, different bonding methods are usually used for the two steps. Compliant bonding can advantageously be used for the first step of bonding the device leads to the substrate, whereas direct bonding, wherein the bonding tool contacts the leads directly, can advantageously be used for bonding the external leads to the substrate. The external leads are typically fabricated as part of a lead frame that comprises connecting portions for holding the external leads in position during bonding. After bonding, the connecting portions of the lead frame are severed and discarded.
It would be advantageous to combine two bonding steps, such as those described above, into one step, while maintaining the individual characteristics of the separate bonding steps.